The present invention relates to a progressive spectacle lens having two aspherical and in particular progressive surfaces.
Progressive spectacle lenses (also referred to as smooth-transition lenses, multifocal lenses etc.) are normally understood to mean spectacle lenses which, in the region through which the spectacle wearer views an object located at a greater distance—hereinafter referred to as the distance portion or distance vision portion—have a different (lower) refractive power than in the region (near vision portion) through which the spectacle wearer views a near object. Between the distance vision portion and the near vision portion, is what is known as the progression zone, in which the effect of the spectacle lens rises continuously from the action of the distance portion to that of the near portion. The value of the rise in the effect (in diopter) between what is known as the distance reference point and what is known as the near reference point is also designated the addition (Add.). Typical values of the addition lie between about 0.75 diopter and about 3.5 diopter.
As a rule, the distance vison portion is arranged in the upper part of the spectacle lens and designed for the view “to infinity”, while the near vision portion is arranged in the lower region and is designed in particular for reading (distances of 0.33 to 0.4 m). For special applications—for example, pilot spectacles or spectacles for video display workstations—the distance and/or the near vision portion can also be arranged in another way and/or designed for different distances. Furthermore, it is possible for there to be a plurality of near vision portions and/or distance vision portions and correspondingly a plurality of progression zones.
In progressive spectacle lenses having a constant refractive index, in order to achieve an increase in the refractive power between the distance vison portion and the near vision portion, it is necessary for the curvature of one or both lens surfaces to change continuously from the distance portion to the near portion. This means that the surface(s) must be capable of being differentiated continuously at least twice.
The surfaces of spectacle lenses are normally characterized by what are known as the main radii of curvature R1 and R2 at each point on the surface. (Sometimes, instead of the main radii of curvature, what are known as the main curvatures K1=1/R1 and K2=1/R2 are also specified.) The main radii of curvature, together with the refractive index n of the spectacle lens material at each point of the surface, determine the variables frequently used for the ophthalmic characterization of a surface:Surface optical power=0.5*(n−1)*(1/R1+1/R2)Surface astigmatism=(n−1)*(1/R1−1/R2)The surface optical power is the variable via which the increase in the effect from the distance portion to the near portion is achieved. The surface astigmatism (clearly a cylinder effect) is a “disruptive property”, since an astigmatism—if the eye does not itself have an astigmatism to be corrected—which exceeds a value of about 0.5 diopter leads to an image which is perceived as unsharp on the retina.
The change in the curvature of the surface required in order to achieve the increase in the surface optical power, without seeing “disruptive” surface astigmatism can certainly be achieved relatively simply along a (straight or curved) line, but beside this line the result is severe “mixing” of the surface, which leads to high surface astigmatism, which makes the lens worse to a greater or lesser extent in the regions beside the aforementioned line.
For reasons based on surface theory, it is therefore not possible, in a surface whose surface optical power increases from distance portion to near portion, to “keep” the regions beside an (astigmatism-free or afflicted with a predefined astigmatism) line free of physiologically disturbing surface astigmatism. For this purpose, reference is also made to what is known as the Minkwitz formulation.
Since, in the distance portion, the optical effect and therefore the main radii of curvature should not change (practically), it is relatively simple to configure the distance portion of a progressive surface in such a way that the distance portion exhibits a very small surface astigmatism (<0.5 diopter) or even the surface astigmatism value “0” in a large region, that is to say is configured spherically. On the other hand, the “quality” of the configurations of the lateral regions of the transition region is of critical importance for the tolerability of the spectacle lens for the respective spectacle wearer.
The fundamental task in the design of each progressive spectacle lens is therefore, without unreasonable impairment of the distance portion or its size, to configure the side regions in the transition zone and, optionally the side regions of the near portion, in such a way that the spectacle lens is as compatible as possible for the spectacle wearer and in particular for a young presbyope who is using a progressive spectacle lens for the first time.
In the past in order to achieve this fundamental task in the design of a surface of a progressive spectacle lens contributing to a change in the refractive power, the starting point has been a line lying in a plane or a curved line—referred to as the main meridian or the main line—as the “design backbone of the surface”. This line or this construction backbone normally extends approximately centrally on the surface from top to bottom and, with its course, follows approximately the point at which the visual rays pierce the respective spectacle lens surface during a viewing movement and in particular when the eyes are lowered.
The main curvatures of each point on this line are chosen in such a way that the desired increase in the surface optical power from the distance portion to the near portion is achieved. Starting from this line, the side regions of the surface have then been calculated more or less suitably.
A large number of solutions have been disclosed for configuring the side regions. In the initial period of the calculation of progressive spectacle lenses, optimization of only the progressive surface based on pure surface theory was carried out, in which the most far-reaching reduction in the disruptive surface astigmatism or “pushing off” of the surface astigmatism into the lower lateral regions of the spectacle lens lay in the foreground.
For some years, most large manufacturers of progressive spectacle lenses have not optimized the progressive surface from points of view based on pure surface theory, but instead based on what is known as the position of use, that is to say in particular taking account of the astigmatism of oblique beams, so that not only the surface astigmatism but also the total astigmatism is viewed as one relevant variable to be optimized.
In order to calculate a progressive surface in the position of use, a situation of use is defined. This relates either to an actual user, for whom the individual parameters—such as pupil spacing, forward inclination, cornea vertex spacing and so on—are determined specifically in the respective situation of use, and the progressive surface is calculated and produced separately, or to average values, such as are described, for example, in DIN 58 208, Part 2. Information regarding the calculation of the lens surface, particularly in relation to the parameters to be taken into account, is described in U.S. Pat. No. 6,685,316 (=WO 01/57584) the entire disclosure of which is incorporated herein by reference.
Irrespective of whether, in a progressive spectacle lens with only one progressive surface, this surface has been optimized only from points of view of surface theory or for an actual position of use, the result is that only one surface contributes to the increase in the refractive power, there are restrictions with regard to the properties of the optimized surface and therefore of the entire spectacle lens.
Therefore, for a long time, at least in the patent literature, spectacle lenses with two progressive surfaces have been proposed. Examples of known spectacle lenses with two progressive surfaces are described, inter alia, in published German patent application nos. DE 33 31 757 A1 and DE 33 31 763 A1.
The use of two progressive surfaces, that is to say surfaces which contribute to the rise in the optical effect from the distance portion to the near portion, has in any case the advantage that each of the surfaces has-to provide only a part of the addition. Since the image errors of a surface, such as the surface astigmatism or the distortion, increase at a greater than linear rate with the increase in the addition Add. in the usual surface designs, better optical properties already result from dividing up the addition to two surfaces, as compared with a progressive lens with the same distance portion effect and the same addition which has only one progressive surface and whose other surface is a spherical or toric surface.
This statement also applies when the other surface is an aspherical or atoric surface which has been calculated individually in order to adapt a progressive surface which has been optimized for an average situation of use to a specific situation of use which differs from the design situation of use.
In addition, the surface astigmatism values of the eye-side surface and the front surface add up geometrically or in accordance with the crossed cylinder method, that is to say not in terms of magnitude but taking account their axial position. Since, in addition, in the case of the front surface and the eye-side surface, which have been optimized successively or simultaneously while taking account of the already optimized or the respective other surface, the maximum values of the surface astigmatism generally lie at points which are not “pierced” by one and the same visual ray and, in addition, the axial positions of the surface astigmatism are likewise generally different, the (geometrically) added surface astigmatism values of the progressive front surface and of the progressive eye-side surface do not attain the surface astigmatism values of a spectacle lens with only one progressive surface. In addition, it is even possible to configure the surfaces in such a way that the image errors, such as the undesired surface astigmatism values of the two surfaces, at least partly compensate each other. See, for example, the two published German patent applications cited above.
In prior art progressive spectacle lenses having two progressive surfaces, the two surfaces have been calculated only from the point of view of improving the optical properties as compared with progressive spectacle lenses having only one progressive surface.
Optimization of the two progressive surfaces from other points of view, such as in particular geometric points of view, such as, for example, the course of the edge of the front surface of a bordered spectacle lens that is adapted to a lens ring of a spectacle frame, has not been considered in the past. Even the reduction in the mixing mentioned in the aforementioned published German patent application no. DE 33 31 757 A1 is used merely to reduce the surface astigmatism and therefore to improve the optical properties and not to optimize a progressive spectacle lens having two progressive surfaces while taking into account geometric and, in particular, cosmetic points of view.